1. Technical Field
The present invention relates to improvements in a tool condition and failure monitoring apparatus and method. More particularly, the present invention relates to an apparatus for monitoring tool condition by acoustic emission analysis and predicting of imminent tool failure prior to failure in cutting tool applications such as turning machines and other cutting tool applications.
2. Background Art
Cutting tools for turning machines are generally made of tool steel or carbides (with or without a coating) and have a finite life before failure, frequently by breaking. Avoiding such failure is desirable to protect the workpiece being machined and the machine itself from damage which can be caused by the workpiece. A workpiece being machined with a broken tool can be torn from the chuck jaws and do substantial damage to the machine.
One approach that has been suggested is that each tool be used for a predetermined period, either in time or number of workpieces, then changed automatically without regard to appearance, quality of workpiece, or other factors.
In U.S. Pat. No. 4,296,364 tools are adjusted based upon the frequency or time of use of the machine. When the tools can no longer be adjusted in such a system, they are changed, a crude system of tool wear monitoring without respect to condition.
Unfortunately, due to differences in the cutting tool microstructure and the methods used in the manufacture of such cutting tools, cutting tool life varies significantly from one cutting tool to the next, even in the same manufacturing batch. Other than for gross deficiencies (such as improper shape or lack of colored coating), it is difficult, if not impossible, to predict cutting tool life during manufacture or inspection.
In some manufacturing environments, the turning machine opertor personally determines when a cutting tool has reached the end of its useful life. This is accomplished through either visual means (watching for tell-tale signs from the cutting operation, such as sparks, or from the surface, such as deteriorating surface finish) or audible means (listening for "groans" from the machine.) This approach to sensing tool wear or failure requires an experienced operator for the turning machine, making it labor intensive, and reliant upon learned skills which may vary from one operator to the next. Since relatively long periods may pass without tool failure, the operator may not be attentive to changed conditions and miss recognizing the failure in time. Today's manufacturing techniques emphasize reducing the number of manufacturing workers and the special skills required of them, making operator determination of tool failure undesirable, as well as unnecessarily expensive.
Other systems rely on monitoring power required for a cutting operation, perhaps in comparison with historical power level requirements. One such system is disclosed in U.S. Pat. No. 4,207,567. The theory is that, when the tool fails or is about to fail, significant power deviations, either increases or decreases, are indicative of tool failure. Experience indicates that such methods are only partially successful in accurately predicting tool failure in a timely fashion.
The prior art has suggested that cutting-tool life or imminent failure can be predicted by the use of acoustic emissions detecting a characteristic voltage level (or threshold) signal which is emitted prior to failure. This threshold detection system is unreliable in that some tool failures are not predicted, and for other tools, failures are prematurely predicted.
This threshold detecting system also fails to account for a "shoulder effect" where machining near a workpiece shoulder causes high acoustic emissions as a result of the shoulder, not as a result of imminent tool failure. A threshold detection system frequently falsely triggers due to incorrectly interpreting a shoulder effect.
Various articles have been presented regarding tool wear sensing through the use of acoustic emission analysis. An article "Tool Wear Sensing Via Acoustic Emission Analysis" in Proceedings of the 8th North American Manufacturing Research Conference by David A. Dornfeld teaches that RMS monitoring of a continuous acoustic emission is preferred over other techniques such as count or count rate for such tool wear monitoring.
An article entitled "Quantitative Relationships for Acoustic Emission From Orthogonal Metal Cutting", authored by Kannatey-Asibu and Dornfeld in the Transactions of the ACME, August 1981 at page 330, discloses various techniques for processing acoustic emission signals.
An article "Acoustic Emission-3, The Use of Ringdown Counting" by Brindley, Holt and Palmer from Non Destructive Testing, Page 299, January 1974, describes another system for employing an acoustic emission signal using cumulative or total count or acoustic emissions as an indication of failure.
An article entitled "Detection for Cutting Tool Fracture by Acoustic Emission Measurement" by Moriwaki from ANNALS OF THE CIRP, Vol. 29, No. 1, 1980, pp. 35-40 suggests that an absolute or cumulative count of pulses exceeding the predetermined threshold be used for predicting tool failure.
Although there are several methods suggested in the prior art for predicting tool wear failure, none are entirely satisfactory in terms of accurately predicting imminent tool failure at the end of a cutting tool's life.